Silicon photovoltaic element and fabrication method

ABSTRACT

A method of forming a photovoltaic device that includes providing an absorption layer of a first crystalline semiconductor material having a first conductivity type, and epitaxially growing a second crystalline semiconductor layer of a second conductivity type that is opposite the first conductivity type. The first conductivity type may be p-type and the second conductivity type may be n-type, or the first conductivity type may be n-type and the second conductivity type may be p-type. The temperature of the epitaxially growing the second crystalline semiconductor layer does not exceed 500° C. Contacts are formed in electrical communication with the absorption layer and the second crystalline semiconductor layer.

BACKGROUND

The present disclosure relates to photovoltaic devices, and moreparticularly to photovoltaic devices such as, for example, solar cells.

A photovoltaic device is a device that converts the energy of incidentphotons to electromotive force (e.m.f.). Typical photovoltaic devicesinclude solar cells, which are configured to convert the energy in theelectromagnetic radiation from the Sun to electric energy. Each photonhas an energy given by the formula E=hν, in which the energy E is equalto the product of the Plank constant h and the frequency ν of theelectromagnetic radiation associated with the photon.

A photon having energy greater than the electron binding energy of amatter can interact with the matter and free an electron from thematter. While the probability of interaction of each photon with eachatom is probabilistic, a structure can be built with a sufficientthickness to cause interaction of photons with the structure with highprobability. When an electron is knocked off an atom by a photon, theenergy of the photon is converted to electrostatic energy and kineticenergy of the electron, the atom, and/or the crystal lattice includingthe atom. The electron does not need to have sufficient energy to escapethe ionized atom. In the case of a material having a band structure, theelectron can merely make a transition to a different band in order toabsorb the energy from the photon.

The positive charge of the ionized atom can remain localized on theionized atom, or can be shared in the lattice including the atom. Whenthe positive charge is shared by the entire lattice, thereby becoming anon-localized charge, this charge is described as a hole in a valenceband of the lattice including the atom. Likewise, the electron can benon-localized and shared by all atoms in the lattice. This situationoccurs in a semiconductor material, and is referred to asphotogeneration of an electron-hole pair. The formation of electron-holepairs and the efficiency of photogeneration depend on the band structureof the irradiated material and the energy of the photon. In case theirradiated material is a semiconductor material, photogeneration occurswhen the energy of a photon exceeds the band gap energy, i.e., theenergy difference of a band gap of the irradiated material.

The direction of travel of charged particles, i.e., the electrons andholes, in an irradiated material is sufficiently random, and may bereferred to as carrier “diffusion”. Thus, in the absence of an electricfield, photogeneration of electron-hole pairs merely results in heatingof the irradiated material. However, an electric field can break thespatial direction of the travel of the charged particles to harness theelectrons and holes formed by photogeneration.

One exemplary method of providing an electric field is to form a p-n orp-i-n junction around the irradiated material. Due to the higherpotential energy of electrons (corresponding to the lower potentialenergy of holes) in the p-doped material with respect to the n-dopedmaterial, an electric field is generated from the direction of then-doped region toward the p-doped region. Electrons generated in theintrinsic and p-doped regions drift towards the n-doped region due tothe electric field, and holes generated in the intrinsic and n-dopedregions drift towards the p-doped region. Thus, the electron-hole pairsare collected systematically to provide positive charges at the p-dopedregion and negative charges at the n-doped region. The p-n or p-i-njunction forms the core of this type of photovoltaic device, whichprovides electromotive force that can power a device connected to thepositive node at the p-doped region and the negative node at the n-dopedregion.

BRIEF SUMMARY

In one embodiment, a method of forming a photovoltaic device is providedthat employs a low temperature chemical vapor deposition (CVD) method toprovide an epitaxial crystalline semiconductor layer on the absorptionlayer of the photovoltaic device. In one embodiment, the method offorming the photovoltaic device may begin with providing at least anabsorption layer of a first crystalline semiconductor material having afirst conductivity type. A second crystalline semiconductor layer maythen be epitaxially grown on the absorption layer, in which the secondcrystalline semiconductor layer has a second conductivity type that isopposite the first conductivity type. The temperature of the epitaxiallygrowth process is less than 500° C. Contacts may be formed in electricalcommunication with the absorption layer and the second crystallinesemiconductor layer.

In another aspect, the present disclosure provides a photovoltaicdevice, such as a solar cell. In one embodiment, the photovoltaic deviceincludes an absorption layer composed of a crystalline semiconductormaterial. The crystalline semiconductor material of the absorption layeris doped to a first conductivity type. An epitaxial semiconductormaterial is in direct contact with the absorption layer. The epitaxialsemiconductor material is doped to a second conductivity type thatopposite the first conductivity type of the absorption layer. Apassivation layer composed of an intrinsic amorphous semiconductormaterial is in direct contact with the epitaxial semiconductor material.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description, given by way of example and notintended to limit the disclosure solely thereto, will best beappreciated in conjunction with the accompanying drawings, wherein likereference numerals denote like elements and parts, in which:

FIG. 1 is a side cross-sectional view of epitaxially growing a secondcrystalline semiconductor layer having a second conductivity type on anabsorption layer of a first crystalline semiconductor material having afirst conductivity type, wherein a back surface field layer is presentunderlying the absorption layer, in accordance with one embodiment ofthe present disclosure.

FIG. 2( a) is a plot of wavelength (nm) of sunlight vs. a normalizednumber of phonons for 5 nm, 10 nm, and 15 nm thicknesses of intrinsicamorphous hydrogenated silicon (i a-Si:H) as employed in a photovoltaicdevice.

FIG. 2( b) is a plot of the thickness of a layer of intrinsic amorphoushydrogenated silicon (i a-Si:H) vs. the % absorption of sunlight of thelayer of intrinsic amorphous hydrogenated silicon (i a-Si:H) as employedin a photovoltaic device.

FIG. 3 is a side cross-sectional view of forming a passivation layercomposed of an intrinsic amorphous semiconductor material on the secondcrystalline semiconductor layer, in accordance with one embodiment ofthe present disclosure.

FIG. 4 is a side cross-sectional view of one embodiment of foaming atransparent conductive material layer on the passivation layer composedof the intrinsic amorphous semiconductor material, in accordance withone embodiment of the present disclosure.

FIG. 5A is a side cross-sectional view of one embodiment of forming afront contact in direct contact with the transparent conductive materiallayer, and a back contact to the back surface field region of theabsorption layer, in accordance with one embodiment of the presentdisclosure.

FIG. 5B is a side cross-sectional view of another embodiment of forminga back contact in electrical communication with the absorption layer,wherein an intrinsic amorphous semiconductor material layer and a dopedamorphous semiconductor material layer are present between theabsorption layer and the back contact, in accordance with the presentdisclosure.

FIG. 5C is a side cross-sectional view of a photovoltaic deviceincluding a localized back contact in electrical communication with theabsorption layer, wherein the localized back contact includes apatterned dielectric material that provides openings to the absorptionlayer, and a metal contact in direct contact with the back surface ofthe absorption layer that is deposited within the openings, inaccordance with one embodiment of the present disclosure.

FIG. 5D is a side cross-sectional view of a photovoltaic deviceincluding a localized back surface field, in accordance with oneembodiment of the present disclosure.

FIG. 6 is a plot of voltage as a function of current density measuredfrom a photovoltaic device under illumination, in accordance with thepresent disclosure.

DETAILED DESCRIPTION

Detailed embodiments of the claimed structures and methods are disclosedherein; however, it is to be understood that the disclosed embodimentsare merely illustrative of the claimed structures and methods that maybe embodied in various forms. In addition, each of the examples given inconnection with the various embodiments are intended to be illustrative,and not restrictive. Further, the figures are not necessarily to scale,some features may be exaggerated to show details of particularcomponents. Therefore, specific structural and functional detailsdisclosed herein are not to be interpreted as limiting, but merely as arepresentative basis for teaching one skilled in the art to variouslyemploy the methods and structures of the present disclosure.

References in the specification to “one embodiment”, “an embodiment”,“an example embodiment”, etc., indicate that the embodiment describedmay include a particular feature, structure, or characteristic, butevery embodiment may not necessarily include the particular feature,structure, or characteristic. Moreover, such phrases are not necessarilyreferring to the same embodiment. Further, when a particular feature,structure, or characteristic is described in connection with anembodiment, it is submitted that it is within the knowledge of oneskilled in the art to affect such feature, structure, or characteristicin connection with other embodiments whether or not explicitlydescribed.

In one embodiment, the present disclosure provides a photovoltaicdevice, and a method of forming the same, in which an epitaxialcrystalline semiconductor layer is foamed on the absorption layer of thephotovoltaic device using a low temperature chemical vapor deposition(CVD) method. Heterojunction with intrinsic thin layer (HIT) solar cellshave reached 23% efficiency in the laboratory and 21% efficiency inproduction. The HIT cells are comprised of thin stacks ofintrinsic/doped hydrogenated amorphous silicon (a-Si:H) serving as thefront (emitter) and back contacts, on an absorption layer composed ofsingle-crystalline silicon (c-Si). Although, the hydrogenated amorphoussilicon provides for low processing temperatures and higher open circuitvoltages, it has been determined that hydrogenated amorphous siliconwhen employed as a component of the emitter of a solar celldisadvantageously absorbs sunlight. Because hydrogenated amorphoussilicon absorbs sunlight, when utilized as the emitter of a solar cell,the hydrogenated amorphous silicon can result in a decreased shortcircuit current when compared to solar cells having a crystallineemitter. In some embodiments, the structures and methods disclosedherein provide a solar cell that reduces light absorption in the emitterregion of the solar cell, while maintaining the advantage of the lowtemperature processing that is typical for forming an emitter regionincluding hydrogenated amorphous silicon as employed in HIT solar cells.

FIGS. 1-4 depict one embodiment of forming a photovoltaic device, suchas a solar cell, in which at least a portion of the emitter component ofthe photovoltaic device is formed by growing doped epitaxial layers byplasma-enhanced chemical vapor deposition (PECVD) at temperatures ofless than 500° C. from a mixture of silane (SiH₄), hydrogen (H₂), anddopant gasses, such as phosphine (PH₃) and diboration (B₂H₆). The dopedepitaxial layers that provide the emitter region of the photovoltaicdevice are typically formed directly on the surface of an absorptionlayer of the photovoltaic device. In some embodiments, the temperaturefor the formation of the epitaxial layers of at least a portion of theemitter component of the photovoltaic device may be less than 200° C.

As used herein, a “photovoltaic device” is a device, such as a solarcell, that produces free electrons-hole pairs, when exposed toradiation, such as light, and results in the production of an electriccurrent. The photovoltaic device typically includes layers of p-typeconductivity and n-type conductivity that share an interface to providea junction. The “absorption layer” of the photovoltaic device is thematerial that readily absorbs photons to generate charge carriers, i.e.,free electrons or holes. A portion of the photovoltaic device, betweenthe front side and the junction is referred to as the “emitter layer”,and the junction is referred to as the “emitter junction”. The emitterlayer may be present atop the absorption layer, in which the emitterlayer has a conductivity type that is opposite the conductivity type asthe absorption layer. In one example, when the Sun's energy in the formof photons collects in the cell layers, electron-hole pairs aregenerated in the material within the photovoltaic device. The emitterjunction provides the required electric field for the collection of thephoto-generated electrons and holes on the p-doped and n-doped sides ofthe emitter junction, respectively. For this reason, and in thisexample, at least one p-type layer of the photovoltaic device mayprovide the absorption layer, and at least one adjacent n-type layer mayprovide the emitter layer.

FIG. 1 depicts one embodiment of epitaxially growing a secondcrystalline semiconductor layer 15 having a second conductivity type onan absorption layer 10 of a first crystalline semiconductor materialhaving a first conductivity type. As used herein, the term “conductivitytype” denotes a semiconductor material being p-type or n-type. In oneembodiment, the second crystalline semiconductor layer 15 is doped to ann-type conductivity type and the absorption layer 10 is doped to ap-type conductivity type. In another embodiment, the second crystallinesemiconductor layer 15 is doped to a p-type conductivity type and theabsorption layer 10 is doped to an n-type conductivity type.

The absorbing layer 10 may be composed of a crystalline semiconductormaterial. In one embodiment, the crystal structure of the crystallinesemiconductor material is of a single crystal crystalline structure. Theterm “single crystal crystalline structure” denotes a crystalline solid,in which the crystal lattice of the entire sample is substantiallycontinuous and substantially unbroken to the edges of the sample, withsubstantially no grain boundaries. In another embodiment, thecrystalline semiconductor material of the absorption layer 10 is of amulti-crystalline or polycrystalline structure. Contrary to a singlecrystal crystalline structure, a polycrystalline structure is a form ofsemiconductor material made up of randomly oriented crystallites andcontaining large-angle grain boundaries, twin boundaries or both.Multi-crystalline is widely referred to a polycrystalline material withlarge grains (of the order of millimeters to centimeters). Other termsused are large-grain polycrystalline, or large-grain multi-crystalline.The term polycrystalline typically refers to small grains (hundreds ofnanometers, to hundreds of microns). The crystalline semiconductormaterial of the absorption layer 10 is typically a silicon containingmaterial. In one embodiment, the absorption layer 10 is composed of atleast one of Si, Ge, SiGe, SiC, and SiGeC. In yet another embodiment,the absorption layer 10 may be a compound semiconductor, such as a typeIII-IV semiconductors, e.g., GaAs. In one example, the crystallinesemiconductor material of the absorption layer 10 is composed of singlecrystal Si. In one embodiment, the absorption layer 10 of the firstcrystalline semiconductor material having a first conductivity type hasa thickness ranging from 50 nm to 1 mm. In another embodiment, theabsorption layer 10 of the first crystalline semiconductor materialhaving a first conductivity type has a thickness ranging from 1 μm to500 μm.

The first conductivity type of the crystalline semiconductor materialthat provides the absorption layer 10 may be provided by a p-type dopantor an n-type dopant. As used herein, “p-type” refers to the addition ofimpurities to an intrinsic semiconductor that creates deficiencies ofvalence electrons (i.e. holes). In a silicon containing absorption layer10, examples of p-type dopants, i.e., impurities, include but are notlimited to, boron, aluminum, gallium and indium. In one embodiment, inwhich the first conductivity type of the crystalline semiconductormaterial of the absorbing layer 10 is p-type, the p-type dopant ispresent in a concentration ranging from 1×10⁹ atoms/cm³ to 1×10²⁰atoms/cm³. In another embodiment, in which the first conductivity typeis p-type, the p-type dopant is present in a concentration ranging from1×10¹⁴ atoms/cm³ to 1×10¹⁹ atoms/cm³. As used herein, “n-type” refers tothe addition of impurities that contributes free electrons to anintrinsic semiconductor. In a silicon containing absorption layer 10,examples of n-type dopants, i.e., impurities, include but are notlimited to, antimony, arsenic and phosphorous. In one embodiment, inwhich the first conductivity type of the crystalline semiconductormaterial of the absorbing layer 10 is n-type, the n-type dopant ispresent in a concentration ranging from 1×10⁹ atoms/cm³ to 1×10²⁰atoms/cm³. In another embodiment, in which the first conductivity typeis n-type, the n-type dopant is present in a concentration ranging from1×10¹⁴ atoms/cm³ to 1×10¹⁹.

The dopant concentration that provides the first conductivity type ofthe first crystalline semiconductor material that provides theabsorption layer 10 may be graded or uniform. By “uniform” it is meantthat the dopant concentration is the same throughout the entirethickness of the first crystalline semiconductor material that providesthe absorption layer 10. For example, a absorption layer 10 having auniform dopant concentration may have the same dopant concentration atthe upper surface and bottom surface of the first crystallinesemiconductor material that provides the absorption layer, as well asthe same dopant concentration at a central portion of the firstcrystalline semiconductor material between the upper surface and thebottom surface of the absorption layer 10. By “graded” it is meant thatthe dopant concentration varies throughout the thickness of theabsorption layer 10. For example, an absorption layer 10 having a gradeddopant concentration may have an upper surface with a greater dopantconcentration than the bottom surface of the absorption layer 10, andvice versa. In another example, the greatest dopant concentration of thefirst crystalline semiconductor material that provides the absorptionlayer 10 may be present in a central portion of the absorption layer 10between the upper surface and the bottom surface of the absorption layer10. In some embodiments, the dopant gas flow ratio may be varied duringepitaxial growth via plasma enhanced chemical vapor deposition toprovide an absorption layer 10 having a graded dopant concentration.

In one example, the absorption layer 10 has a thickness ranging from 2nm to 100 nm, and the first conductivity type is provided by a p-typedopant of boron is present in a concentration ranging from 10¹⁴atoms/cm³ to 10¹⁹ atoms/cm³.

The band gap of the absorption layer 10 can be from 0.1 eV to 7.0 eV.

In one embodiment, a back surface field layer 5 is present underlyingthe absorption layer 10. The back surface field layer 5 can serve topassivate the back surface of the absorption layer 10, and reduceelectron-hole recombination. The back surface field layer 5 is typicallydoped to the same conductivity type, i.e., first conductivity type, asthe absorption layer 10. Therefore, in the embodiments in which thefirst conductivity type of the crystalline semiconductor material of theabsorption layer 10 is p-type, the back surface field layer 5 is dopedto provide a p-type conductivity, and in the embodiments in which thefirst conductivity type of the crystalline semiconductor material of theabsorption layer 10 is n-type, the back surface field layer 5 is dopedto provide an n-type conductivity.

The back surface field layer 5 may be composed of the same or adifferent material as the crystalline semiconductor material of theabsorption layer 10. Typically, the back surface field layer 5 iscomposed of a silicon containing material, such as Si, Ge, SiGe, SiC,SiGeC and combinations thereof. The back surface field layer 5 may alsobe a compound semiconductor, such as type III-IV semiconductors, e.g.,GaAs. The back surface field layer 5 can have the same or differentcrystal structure as the crystalline semiconductor material thatprovides the absorption layer 10. For example, the back surface fieldlayer 5 may have a single crystal crystalline structure or apolycrystalline crystal structure. In one example, the back surfacefield layer 5 is composed of single crystal Si. In some embodiments, theback surface field layer 5 can be an epitaxial crystalline layer. Anepitaxial crystalline layer is a material layer having the samecrystallographic characteristics, such as crystal structure, as thematerial layer on which it is formed. For example, the back surfacefield layer 5 may be an epitaxial layer 5 having the same crystalstructure as the absorption layer 10, on which the back surface fieldlayer 5 is grown. In one embodiment, the back surface field layer 5 hasa thickness ranging from 2 nm to 10 μm. In another embodiment, the backsurface field layer 5 has a thickness ranging from 5 nm to 5 μm.

The dopant that dictates the conductivity type of the back surface fieldlayer 5 is typically present in the back surface field layer 5 in agreater concentration than the dopant that dictates the concentrationtype of the crystalline semiconductor material that provides theabsorption layer 10. In one embodiment, the concentration of the dopantthat provides the conductivity type of the back surface field layer 5typically ranges from 10¹⁷ atom/cm³ to 10²¹ atom/cm³. In anotherembodiment, the concentration of the dopant that provides theconductivity type of the back surface field layer 5 ranges from 10¹⁹atom/cm³ to 10²⁰ atom/cm³. The back surface field layer 5 is optional,and may be omitted from the photovoltaic device of the presentdisclosure.

The absorption layer 10 and the back surface field layer 5 may be formedfrom a semiconductor substrate. In one embodiment, the semiconductorsubstrate can provide the absorption layer 10, and the optionally backsurface field layer 5 can be deposited onto the semiconductor substrate.For example, the back surface field layer 5 can be deposited onto thesemiconductor substrate that provides the absorption layer 10 usingchemical vapor deposition (CVD). CVD is a deposition process in which adeposited species is fowled as a result of chemical reaction betweengaseous reactants, wherein the solid product of the reaction isdeposited on the surface on which a film, coating, or layer of the solidproduct is to be formed. Variations of CVD processes suitable fordepositing the back surface field layer 5 include, but are not limitedto, Atmospheric Pressure CVD (APCVD), Low Pressure CVD (LPCVD) andPlasma Enhanced CVD (PECVD), Metal-Organic CVD (MOCVD) and combinationsthereof. In one example, the back surface field layer 5 may be formedusing a low temperature PECVD process similar to the low temperaturePECVD process that epitaxially forms the second crystallinesemiconductor layer 15 on the absorption layer 10. The details of thislow temperature PECVD process are described below. In anotherembodiment, the back surface field layer 5 can be formed into thesemiconductor substrate that provides the absorption layer 10 using adiffusion method. For example, the dopant that provides the conductivitytype of the back surface field layer 5 may be introduced to thesemiconductor substrate using ion implantation or gas vapor phasedeposition, and then diffused into a depth of the semiconductorsubstrate to provide the back surface field layer 5. The methods bywhich the absorption layer 10 and the back surface field layer 5 areformed are provided for illustrated purposes only, and are not intendedto limit the present disclosure.

Still referring to FIG. 1, the second crystalline semiconductor layer 15having the second conductivity type may be epitaxially grown on theabsorption layer 10 of the first crystalline semiconductor materialhaving the first conductivity type. The second crystalline semiconductorlayer 15 provides at least a component of the emitter of the solar cell.The second crystalline semiconductor layer 15 may be composed of asilicon containing material, such as Si, Ge, SiGe, SiC, SiGeC andcombinations thereof. The second crystalline semiconductor layer 15 mayalso be a compound semiconductor, such as type III-IV semiconductors,e.g., GaAs. The second conductivity type of the second crystallinesemiconductor layer 15 is opposite the conductivity type of theabsorption layer 10. Therefore, when the absorption layer 10 has ap-type conductivity, the second crystalline semiconductor layer 15 hasan n-type conductivity, and when the absorption layer 10 has an n-typeconductivity, the second crystalline semiconductor layer 15 has a p-typeconductivity.

In a silicon containing second crystalline semiconductor layer 15,examples of p-type dopants, i.e., impurities, include but are notlimited to, boron, aluminum, gallium and indium. In one embodiment, inwhich the second conductivity type of the second crystallinesemiconductor layer 15 is p-type, the p-type dopant is present in aconcentration ranging from 10¹⁶ atoms/cm³ to 10²¹ atoms/cm³. In anotherembodiment, in which the second conductivity type of the secondcrystalline semiconductor layer 15 is p-type, the p-type dopant ispresent in a concentration ranging from 10¹⁸ atoms/cm³ to 5 10²⁰atoms/cm³. In a silicon containing second crystalline semiconductorlayer 15, examples of n-type dopants, i.e., impurities, include but arenot limited to, antimony, arsenic and phosphorous. In one embodiment, inwhich the second conductivity type is n-type, the n-type dopant ispresent in a concentration ranging from 10¹⁶ atoms/cm³ to 10²¹atoms/cm³. In another embodiment, in which the second conductivity typeis n-type, the n-type dopant is present in a concentration ranging from10¹⁸ atoms/cm³ to 5 10²⁰ atoms/cm³.

The thickness of the second crystalline semiconductor layer 15 may rangefrom 2 nm to 2 μm. In another embodiment, the thickness of the secondcrystalline semiconductor layer 15 ranges from 3 nm to 500 nm.Typically, as the thickness of the emitting layer 15 decreases thedopant concentration that dictates the conductivity type of the secondcrystalline layer 15 is increased. In one example, the secondcrystalline semiconductor layer 15 has a thickness ranging from 5 nm to15 nm, and the second conductivity type is provided by an n-type dopantof phosphorus that is present in a concentration ranging from 10¹⁸atoms/cm³ to 2×10¹⁹ atoms/cm³.

The band gap of the second crystalline semiconductor layer 15 can befrom 0.5 eV to 2.0 eV.

As indicated above, the second crystalline semiconductor layer 15 havingthe second conductivity type is epitaxially grown on the absorptionlayer 10. “Epitaxial growth and/or deposition” means the growth of asemiconductor material on a deposition surface of a semiconductormaterial, in which the semiconductor material being grown has the same(or nearly the same) crystalline characteristics as the semiconductormaterial of the deposition surface. Therefore, in the embodiments inwhich the first crystalline material of the absorption layer 10 has asingle crystal crystalline structure, the second crystallinesemiconductor layer 15 that is epitaxially grown on the absorption layer10 will also have a single crystal crystalline structure. Further, inthe embodiments in which the first crystalline material of theabsorption layer 10 has a polycrystalline or multi-crystallinestructure, the second crystalline semiconductor layer 15 that isepitaxially grown on the absorption layer 10 will also have apolycrystalline or multi-crystalline structure.

The second crystalline semiconductor layer 15 replaces the intrinsicamorphous hydrogenated silicon (a-Si:H) that is typically formed on theabsorption layer of prior HIT solar cells. The term “amorphous” denotesthat the intrinsic amorphous hydrogenated silicon lacks a specificcrystal structure. By replacing the intrinsic amorphous hydrogenatedsilicon with the second crystalline semiconductor layer 15, the presentdisclosure decreases the absorption of phonons of sunlight before thephonons of sunlight are converted into charge carriers. FIGS. 2( a) and2(b) illustrate the absorption effect of intrinsic amorphoushydrogenated silicon as employed in a solar cell. FIG. 2( a) is a plotof wavelength (nm) of sunlight vs. normalized number of phonons, inwhich the data line identified by reference number 1 illustrates theabsorption of a HIT solar cell including a 5 nm thick layer of intrinsicamorphous hydrogenated silicon, the data line identified by referencenumber 2 illustrates the absorption of an HIT solar cell including a 10nm thick layer of intrinsic amorphous hydrogenated silicon, and the dataline identified by reference number 3 illustrates the absorption of anHIT solar cell including a 15 nm thick layer of intrinsic amorphoushydrogenated silicon. The data line identified by reference number 4 isthe absorption measured from a control solar cell in which an intrinsicamorphous hydrogenated silicon is not present. FIG. 2( b) is a plot ofthe thickness of a layer of intrinsic amorphous hydrogenated silicon (ia-Si:H) vs. the % absorption of sunlight of the layer of intrinsicamorphous hydrogenated silicon as employed in a solar cell. Asillustrated in FIGS. 2( a) and 2(b), phonon absorption by the intrinsicamorphous hydrogenated silicon increases as the thickness of the layerof the intrinsic amorphous hydrogenated silicon increases.

By replacing the intrinsic amorphous hydrogenated silicon with thesecond crystalline semiconductor layer 15 to provide at least a portionof the emitter component of the solar cell, the present disclosuresubstantially reduces phonon absorption in the emitter before thephonons of sunlight are converted into charge carriers. For example, anemitter including the second crystalline semiconductor layer 15 havingthe second conductivity type that is foamed in accordance with thepresent disclosure reduces phonon absorption in the emitter component ofthe solar cell by 1% to 10% when compared to a similarly structured HITsolar cell having an emitter composed of intrinsic amorphoushydrogenated silicon.

Although the present disclosure does not employ intrinsic amorphoushydrogenated silicon as a component of the emitter of the solar cell,the present disclosure does enable the low temperature processing thatis consistent with the temperatures used in foaming intrinsic amorphoushydrogenated silicon for HIT solar cells. More specifically, the plasmaenhanced chemical vapor deposition (PECVD) method of present disclosureallows for the second crystalline semiconductor layer 15 to beepitaxially formed on the absorption layer 10 at temperatures of lessthan 500° C., e.g., less than 200° C. The temperatures disclosed hereinfor the epitaxial growth of the second crystalline semiconductor layer15 are measured at the deposition surface, and may also be referred toas substrate temperatures. Plasma enhanced chemical vapor deposition(PECVD) is a deposition process used to deposit films from a gas state(vapor) to a solid state on a deposition substrate. Chemical reactionsare involved in the process, which occur after creation of a plasma ofthe reacting gases. A plasma is any gas in which a significantpercentage of the atoms or molecules are ionized. Fractional ionizationin plasmas used for deposition and related materials processing variesfrom about 10⁻⁴ in capacitive discharge plasmas to as high as 5-10% inhigh density inductive plasmas. Processing plasmas are typicallyoperated at pressures of a few millitorr to a few torr, although arcdischarges and inductive plasmas can be ignited at atmospheric pressure.In some embodiments, the plasma is created by RF (AC) frequency, such asa radio frequency induced glow charge, or DC discharge between twoelectrodes, the space between which is filled with the reacting gases.In one example, a PECVD device employs a parallel plate chamberconfiguration.

The second crystalline semiconductor layer 15 is epitaxially grown viaplasma enhanced chemical vapor deposition (PECVD) from a mixture ofsilane (SiH₄), hydrogen (H₂) and dopant gasses. Other gases such asSiF₄, GeH₄ and CH₄ may be used for growing c-Ge (crystalline germanium),c-SiGe, or incorporating C into the c-Si (crystalline silicon), c-Ge(crystalline germanium) or c-SiGe (crystalline silicon germanium) film.In one embodiment, to provide epitaxial growth of a second crystallinesemiconductor layer 15 composed of a silicon containing material anddoped to a second conductivity, e.g., n-type conductivity or p-typeconductivity, at temperatures of less than 500° C., the ratio of silaneprecursor gas (SiH₄) to hydrogen gas (H₂) is selected to be greater than5:1. In another embodiment, the ratio of silane (SiH₄) to hydrogen (H₂)ranges from 5:1 to 1000:1. For example, epitaxial growth of silicon ispossible at temperatures as low as 150° C. with ratios of silane (SiH₄)to hydrogen (H₂) ranging from 5:1 to 20:1.

The dopant gasses of the low temperature PECVD process provide theconductivity type of the second crystalline semiconductor layer 15. Morespecifically, as the second crystalline semiconductor layer 15 isepitaxially grown it is in-situ doped. The in-situ doping of the n-typedopants can be effected by adding a dopant gas including at least onep-type dopant, e.g., phosphorus or arsenic, into the gas stream into theprocess chamber. For example, when phosphorus is the n-type dopant, thedopant gas can be phosphine (PH₃), and when arsenic is the n-typedopant, the dopant gas can be arsine (AsH₃). In one example, when thesecond conductivity type dopant is n-type, the dopant gasses includephosphine gas (PH₃) present in a ratio to silane (SiH₄) ranging from0.01% to 10%. In another example, when the second conductivity typedopant is n-type, the dopant gasses include phosphine gas (PH₃) presentin a ratio to silane (SiH₄) ranging from 0.1% to 2%. In one example, asecond crystalline semiconductor layer 15 of single crystal silicon wasepitaxial grown on a single crystal absorbing layer 10 at a temperatureof 150° C., wherein the second crystalline semiconductor layer 15 has ann-type conductivity provided by phosphorus dopant present in aconcentration greater than 1×10²⁰ cm⁻³.

The in-situ doping of p-type dopant to provide the second conductivitytype in the second crystalline semiconductor layer 15 can be effectedwith a dopant gas including at least one p-type dopant, e.g., B, intothe gas stream into the process chamber. For example, when boron is thep-type dopant, the dopant gas can be diborane (B₂H₆). In one embodiment,wherein the second conductivity type dopant is p-type, the dopant gassesfor forming the second crystalline semiconductor layer 15 may bediborane (B₂H₆) present in a ratio to silane (SiH₄) ranging from 0.01%to 10%. In another embodiment, wherein the second conductivity typedopant is p-type, the dopant gasses for forming the second crystallinesemiconductor layer 15 may be diborane (B₂H₆) present in a ratio tosilane (SiH₄) ranging from 0.1% to 2%. In yet another embodiment, inwhich the second conductivity type dopant is p-type, the dopant gassesfor forming the second crystalline semiconductor layer 15 may betrimethylboron (TMB) present in a ratio to silane (SiH₄) ranging from0.1% to 10%.

The dopant that is introduced to the second crystalline semiconductorlayer 15 may be uniform in concentration or may have a gradedconcentration. By “uniform” it is meant that the dopant concentration isthe same throughout the entire thickness of the second crystallinesemiconductor layer 15. For example, a second crystalline semiconductorlayer 15 having a uniform dopant concentration may have the same dopantconcentration at the upper surface and bottom surface of the secondcrystalline semiconductor layer 15, as well as the same dopantconcentration at a central portion of the second crystallinesemiconductor layer 15 between the upper surface and the bottom surfaceof the second crystalline semiconductor layer 15. By “graded” it ismeant that the dopant concentration varies throughout the thickness ofthe second crystalline semiconductor layer 15. For example, a secondcrystalline semiconductor layer 15 having a graded dopant concentrationmay have an upper surface with a greater dopant concentration than thebottom surface of the second crystalline semiconductor layer 15, andvice versa. In another example, the greatest dopant concentration of thesecond crystalline semiconductor layer 15 may be present in a centralportion of the second crystalline semiconductor layer 15 between theupper surface and the bottom surface of the second crystallinesemiconductor layer 15.

In one embodiment, to provide a graded dopant concentration in thesecond crystalline semiconductor layer 15, the gas flow ratio for thedopant gas may be varied during epitaxial growth of the secondcrystalline semiconductor layer by PECVD. In the embodiments, in whichthe second crystalline semiconductor layer 15 that is composed ofsilicon germanium (SiGe) or silicon doped with carbon (Si:C), the carbon(C) or germanium (Ge) dopant may be graded by varying the gas flow ratiofor the gas precursors that provide the carbon and/or germanium duringthe epitaxial growth of the second crystalline semiconductor layer 15.

The pressure for the low temperature PECVD process for epitaxiallygrowing the second crystalline semiconductor layer 15 ranges from 10mTorr to 5 Torr, and in one example may be in the range of 250 mtorr to900 mTorr. The power density for the low temperature PECVD process forepitaxially growing the second crystalline semiconductor layer 15 mayrange from 1 mW/cm² to 100 mW/cm², and in one example may be in therange of 3 mW/cm² to 10 mW/cm². Further details regarding the epitaxialgrowth process for forming the second crystalline semiconductor layer 15of the present disclosure are described in U.S. patent application Ser.No. ______ (IBM Docket No. YOR920110088US1 titled “Ultra Low-temperatureselective epitaxial growth of Silicon for CMOS and 3D integration”),which is owned by the assignee of the present disclosure, and isincorporated herein by reference.

FIG. 3 depicts one embodiment of forming a passivation layer 20 composedof an intrinsic amorphous semiconductor material on the secondcrystalline semiconductor layer 15. The term “intrinsic semiconductor”,also called an undoped semiconductor or i-type semiconductor, is asubstantially pure semiconductor without any significant dopant speciespresent. The number of charge carriers in the intrinsic semiconductor isdetermined by the properties of the material itself instead of theamount of impurities, i.e., dopants. Typically, in intrinsicsemiconductors the number of excited electrons and the number of holesare equal (n=p). The passivation layer 20 can serve to passivate theupper surface of the second crystalline semiconductor layer 15, andreduce electron-hole recombination. The intrinsic amorphoussemiconductor material that provides the passivation layer 20 istypically, but not necessarily always hydrogenated. Typically, thepassivation layer 20 is composed of intrinsic amorphous hydrogenatedsilicon (i a-Si:H). Typically, the thickness of the passivation layer 20is from 100 nm to 1 micron, although lesser and greater thicknesses canalso be employed.

The passivation layer 20 is formed utilizing any physical or chemicalvapor deposition process including any semiconductor precursor sourcematerial. In some embodiments, the intrinsic hydrogenated semiconductorcontaining material is deposited in a process chamber containing asemiconductor precursor source gas and a carrier gas including hydrogen.Hydrogen atoms in the hydrogen gas within the carrier gas areincorporated into the deposited material to form the intrinsichydrogenated semiconductor containing material of the intrinsicsemiconductor layer 20. The passivation layer 20 is optional, and may beomitted.

FIG. 4 depicts one embodiment of forming a transparent conductivematerial layer 25 on the passivation layer 20 composed of the intrinsicamorphous semiconductor material. Throughout this disclosure an elementis “transparent” if the element is sufficiently transparent in thevisible electromagnetic spectral range. The transparent conductivematerial layer 25 includes a conductive material that is transparent inthe range of electromagnetic radiation at which photogeneration ofelectrons and holes occur within the solar cell structure. In oneembodiment, the transparent conductive material layer 25 can include atransparent conductive oxide such as, but not limited to, afluorine-doped tin oxide (SnO₂:F), an aluminum-doped zinc oxide(ZnO:Al), tin oxide (SnO) and indium tin oxide (InSnO₂, or ITO forshort). The thickness of the transparent conductive material layer 25may vary depending on the type of transparent conductive materialemployed, as well as the technique that was used in forming thetransparent conductive material. Typically, and in one embodiment, thethickness of the transparent conductive material layer 25 ranges from 20nm to 500 nm. Other thicknesses, including those less than 20 nm and/orgreater than 500 nm can also be employed. The optimum thickness of TCOfor minimizing reflection from the surface of Si is in the range of 70nm to 110 nm. The transparent conductive material layer 25 is typicallyformed using a deposition process, such as sputtering or CVD. Examplesof CVD processes suitable for forming the transparent conductivematerial layer 25 include, but are not limited to, APCVD, LPCVD, PECVD;MOCVD and combinations thereof. Examples of sputtering are included butnot limited to RF and DC magnetron sputtering.

The top, bottom, or both surfaces of the absorption layer 10, and/or thetop surface of the transparent conductive material layer 25 may betextured. A textured (i.e., specially roughened) surface is used insolar cell applications to increase the efficiency of light absorption.The textured surface decreases the fraction of incident light lost toreflection relative to the fraction of incident light transmitted intothe cell since photons incident on the side of an angled feature will bereflected onto the sides of adjacent angled features and thus haveanother chance to be absorbed. Moreover, the textured surface increasesinternal absorption, since light incident on an angled surface willtypically be deflected to propagate through the device at an obliqueangle, thereby increasing the length of the path taken to reach thedevice's back surface, as well as making it more likely that photonsreflected from the device's back surface will impinge on the frontsurface at angles compatible with total internal reflection and lighttrapping. In one embodiment, the texturing of the transparent conductivematerial layer 25, e.g., TCO, is achieved utilizing a hydrogen based wetetch chemistry, such as, for example, etching in HCl. In someembodiments, the textured upper surface can be achieved duringformation, i.e., deposition, of the transparent conductive materiallayer 25. The transparent conductive material layer 25 is optional, andmay be omitted. In one embodiment, the texturing of thesingle-crystalline Si absorbing layer 10 is achieved utilizing a KOHbased wet etch chemistry to realize random pyramids, or invertedpyramids.

FIG. 5A depicts one embodiment of forming a front contact 35 (alsoreferred to as an emitter contact 35) in direct contact with thetransparent conductive material layer 25, and a back contact 40 inelectrical communication with the absorption layer 10. In the embodimentthat is depicted in FIG. 5A, the back contact 40 is in direct contactwith the back surface field layer 5, but in the embodiments in which theback surface field layer 5 is omitted, the back contact 40 may be indirect contact with the absorption layer 10. In the embodiment that isdepicted in FIG. 5A, the front contact 35 is in direct contact with thetransparent conductive material layer 25, but in the embodiments inwhich the transparent conductive material layer 25 is omitted, the frontcontact 35 may be in direct contact with the passivation layer 20. Inyet another embodiment, in which both the transparent conductivematerial layer 25 and the passivation layer 20 are omitted, the frontcontact 35 may be in direct contact with the second crystallinesemiconductor layer 15.

In one embodiment, the front contact 35 of a solar cell consists of aset of parallel narrow finger lines and wide collector lines depositedtypically at a right angle to the finger lines. The front contact 35 maybe deposited with a screen printing technique. In another embodiment,the front contact 35 is provided by the application of an etched orelectroformed metal pattern. The metallic material used in forming themetal pattern for the front contact 35 may include applying a metallicpaste. The metallic paste may be any conductive paste, such as Al paste,Ag paste or AlAg paste. The metallic material used in forming the metalpattern for the front contact 35 may also be deposited using sputteringor plating. The thickness of the front contact 35 can range from 100 nmto 10 micrometers, although lesser and greater thicknesses can also beemployed. In some embodiments, forming the front contact 35 may includeapplying an antireflection (ARC) coating 36. The antireflection coating(ARC) 36 may be composed of silicon nitride (SiN_(x)) or silicon oxide(SiO_(x)) grown by PECVD at temperatures as low as 200° C. In anotherexample, the antireflective coating (ARC) 36 may be a dual layerstructure composed of zinc-sulfide (ZnS) and magnesium fluoride (MgF₂).

In some embodiments, the back contact 40 is deposited on the optionalback surface field layer 5 or the absorption layer 10. The back contact40 may be blanket deposited using a physical vapor deposition (PVD)method, such as sputtering or plating. The back contact 40 may becomposed of any conductive material, such as aluminum, and may have athickness ranging from 100 nm to 10 micrometers, although lesser andgreater thicknesses can also be employed.

FIG. 5B depicts another embodiment of forming a back contact 41 inelectrical communication with the absorption layer 10, wherein anintrinsic amorphous semiconductor material layer 42 and a dopedamorphous semiconductor material layer 43 are present between theabsorption layer 10 and the back contact 41. The photovoltaic device,e.g., solar cell, depicted in FIG. 5B includes a transparent conductivematerial layer 25, a passivation layer 20, a second crystallinesemiconductor layer 15, and an absorption layer 10 composed of a firstcrystalline semiconductor layer, as described in FIGS. 1-5A. In thisembodiment, the back surface field layer 5 may be omitted. An intrinsicamorphous semiconductor material layer 42 may be present in directcontact with the absorption layer 10. The intrinsic amorphoussemiconductor material layer 42 may be similar in composition and methodof formation as the passivation layer 20, that is described above withreference to FIG. 3. Therefore, the description of the passivation layer20 discussed above with reference to FIG. 3 is suitable for theintrinsic amorphous semiconductor material layer 42 that is depicted inFIG. 5B.

In one embodiment, a doped amorphous semiconductor material layer 43 isin direct contact with the intrinsic amorphous semiconductor materiallayer 42. The doped amorphous semiconductor material 43 may be composedof a silicon containing layer, such as silicon, silicon germanium, orsilicon doped with carbon (Si:C). The doped amorphous semiconductormaterial 43 may be deposited using a chemical vapor deposition (CVD)process, such as plasma enhanced chemical vapor deposition (PECVD).Examples of suitable silicon-containing reactant gas for PECVDdeposition include SiH₄, SiH₂Cl₂, SiHCl₃, SiCl₄, and Si₂H₆. Examples ofgermanium-containing reactant gas for PECVD include GeH₄, GeH₂Cl₂,GeCl₄, and Ge₂H₆. The doped amorphous semiconductor material layer 43may have a conductivity type that is the same as the absorbing layer 10.In one embodiment, the doped amorphous semiconductor layer 43 is dopedto a p-type conductivity that is introduced to the doped amorphoussemiconductor material layer 43 using an in-situ doping process. Thein-situ doping of the p-type dopants into a silicon containing dopedamorphous semiconductor material layer 43 can be effected by adding adopant gas including at least one p-type dopant, e.g., B, In, and Ga,into the gas stream into the process chamber of the PECVD device.

A backside transparent conductive material layer 44 may be in directcontact with the doped amorphous semiconductor material layer 43. Thebackside transparent conductive material layer 44 is similar incomposition and method of manufacturing to the transparent conductivematerial layer 25 that is described above with reference to FIG. 4. Inone embodiment, the metal layer may be substituted for the backsidetransparent conductive material layer 44. The metal layer may be analuminum containing layer that is deposited using a physical vapordeposition (PVD) process, such as sputtering or plating.

Still referring to FIG. 5B, a back contact 41 may then be formed incontact with the backside transparent conductive material layer 44 orthe metal layer. In one embodiment, the back contact 41 may be similarin composition and method of manufacturing as the front contact 35.

FIG. 5C depicts one embodiment of a photovoltaic device including alocalized back contact in electrical communication with the absorptionlayer 10. The localized back contact includes a patterned dielectriclayer 46 that provides openings to the absorption layer 10, and a metalcontact 47 in direct contact with the back surface of the absorptionlayer 10 that is deposited within the openings. The patterned dielectriclayer 46 may function as a passivation layer, and may be composed of anoxide, nitride or oxynitride material, e.g., silicon oxide or siliconnitride. The openings that are formed through the patterned dielectriclayer 46 may be produced using photolithography and etching. The metalcontact 47 may be deposited within the openings in the patterneddielectric layer 46 using a physical vapor deposition (PVD) process,such as sputtering or plating. In one example, the metal contact 47 maybe composed of aluminum, but other metals are suitable for providing themetal contact 47. The photovoltaic device, e.g., solar cell, depicted inFIG. 5C also includes a transparent conductive material layer 25, apassivation layer 20, a second crystalline semiconductor layer 15, andan absorption layer 10 composed of a first crystalline semiconductorlayer, as described in FIGS. 1-5B.

FIG. 5D depicts one embodiment of a photovoltaic device including alocalized back surface field. The photovoltaic device depicted in FIG.5D is similar to the photovoltaic device depicted in FIG. 5C with theexception that the photovoltaic device that is depicted in FIG. 5Dfurther includes a localized back surface field 48. The photovoltaicdevice, e.g., solar cell, depicted in FIG. 5D includes a transparentconductive material layer 25, a passivation layer 20, a secondcrystalline semiconductor layer 15, an absorption layer 10 composed of afirst crystalline semiconductor layer, a patterned dielectric layer 46,and a metal contact 47, as described in FIGS. 1-5C. The localized backsurface field 48 may be provided by a doped region in the absorptionlayer 10. In one embodiment, the doped region that provides thelocalized back surface field 48 has the same conductivity type as theabsorption layer 10. For example, the absorption layer 10 and thelocalized back surface field 48 may both be doped to a p-typeconductivity. Typically, the dopant concentration that provides theconductivity type of the absorption layer 10 and the localized backsurface field 48, is greater in the localized back surface field 48 thanin the absorption layer 10. For example, the dopant concentration of thep-type dopant in the absorption layer 10 may range from 10¹⁸ atoms/cm³to 10²¹ atoms/cm³, and the dopant concentration of the p-type dopant inthe localized back surface field regions 48 may range from 10¹⁹atoms/cm³ to 10²⁰ atoms/cm³. The dopant that provides the localized backsurface field 48 may be introduced through the openings in the patterneddielectric layer 46 into an exposed surface of the absorption layer 10by ion implantation or diffusion, or by the combination thereof, priorto the formation of the metal contact 47.

The following example is given to illustrate the effect of replacing theemitter component of the solar cell that is composed of intrinsicamorphous hydrogenated silicon with the second crystalline semiconductorlayer 15 of the present disclosure, in which the second crystallinesemiconductor layer 15 is epitaxially grown at a temperature of lessthan 500° C. on the absorption layer 10. Because this examples is givenfor illustrative purposes only, the present disclosure should not beinterpreted as being limited thereto.

FIG. 6 is a plot of voltage as a function of current density measuredfrom a photovoltaic device including an emitter composed of the secondcrystalline semiconductor layer 15 having the second conductivity typethat is epitaxially grown at a temperature of less than 500° C. on theabsorption layer 10 of the first crystalline semiconductor material thathas the first conductivity type, as described above with reference toFIG. 1-5A. The data line identified by reference number 45 is measuredfrom a solar cell including a second crystalline semicondcutor layer 15as depicted in FIG. 5A. The data line identified by reference number 50is measured from a control solar cell, in which at least a portion ofthe emitter component of the solar cell includes a intrinsic amorphoushydrogenated silicon. The structure of the control solar cell is similarto the solar cell depicted in FIG. 5A with the exception that the secondcrystalline semiconductor layer 15 is replaced with an intrinsicamorphous hydrogenated silicon layer (serving as a passivation layer),and the passivation layer 20 is replaced with doped a-Si:H—with oppositedoping type to that of the absorbing layer 10—(serving as the emittinglayer). FIG. 2 depicts that the short circuit current density (currentdensity measured at voltage of 0) of the solar cell including the secondcrystalline semicondcutor layer 15 shows an approximately 5% improvementwhen compared to the short circuit current density of the control solarcell. The thickness of the absorbing layer 10 is 2 μm in both solarcells.

While the present disclosure has been particularly shown and describedwith respect to preferred embodiments thereof, it will be understood bythose skilled in the art that the foregoing and other changes in formsand details can be made without departing from the spirit and scope ofthe present disclosure. It is therefore intended that the presentdisclosure not be limited to the exact forms and details described andillustrated, but fall within the scope of the appended claims.

1. A photovoltaic device comprising: an absorption layer comprised of acrystalline semiconductor material, wherein the crystallinesemiconductor material is doped to a first conductivity type; anepitaxial semiconductor material in direct contact with the absorptionlayer, wherein the epitaxial semiconductor material is doped to a secondconductivity type opposite the first conductivity type; and apassivation layer comprises an intrinsic amorphous semiconductormaterial is in direct contact with the epitaxial semiconductor material.2. The photovoltaic device of claim 1, wherein the crystallinesemiconductor material of the absorption layer comprises a singlecrystal crystalline structure.
 3. The photovoltaic device of claim 1,wherein the crystalline semiconductor material of the absorption layercomprises a poly-crystalline or multi-crystalline structure.
 4. Thephotovoltaic device of claim 1, wherein the crystalline semiconductormaterial is a silicon-containing material.
 5. The photovoltaic device ofclaim 4, wherein the first conductivity type is n-type and the secondconductivity type is p-type, or the first conductivity type is p-typeand the second conductivity type is n-type.
 6. The photovoltaic deviceof claim 5, wherein the absorption layer has a thickness ranging from 50nm to 1 mm, and the first conductivity type is provided by a dopantpresent in a concentration ranging from 10⁹ atoms/cm³ to 10²⁰ atoms/cm³.7. The photovoltaic device of claim 1, wherein the epitaxialsemiconductor material is comprises a single crystal crystallinestructure.
 8. The photovoltaic device of claim 1, wherein the epitaxialsemiconductor material is a poly-crystalline or multi-crystallinestructure.
 9. The photovoltaic device of claim 1, wherein the epitaxialsemiconductor material is a silicon-containing material.
 10. Thephotovoltaic device of claim 9, wherein the epitaxial semiconductormaterial has a thickness ranging from 2 nm to 2 μm, and the firstconductivity type is provided by a dopant present in a concentrationranging from 10¹⁶ atoms/cm³ to 5 10²⁰ atoms/cm³.
 11. The photovoltaicdevice of claim 1, further comprising a transparent conductive materiallayer present in direct contact with the passivation layer.
 12. Thephotovoltaic device of claim 11, further comprising an emitter contactin direct contact with the transparent conductive material layer, and aback contact to the absorption layer.
 13. The photovoltaic device ofclaim 12, further comprising a back surface field layer between and indirect contact with the absorption layer and the back contact, whereinat least a portion of the back surface field layer comprises asemiconductor material doped to provide a same conductivity type as theabsorption layer.
 14. The photovoltaic device of claim 13, wherein theback surface field layer comprises single or multi-layers ofcrystalline, or non-crystalline semiconductor material, having athickness ranging from 2 nm to 10 μm, wherein a dopant to provide thesame conductivity type as the absorption layer is present in the backsurface field layer in a concentration ranging from 10¹⁷ atoms/cm³ to10²¹ atoms/cm³.
 15. The photovoltaic device of claim 1, wherein thepassivation layer is comprised of intrinsic amorphous hydrogenatedsilicon (a-Si:H).
 16. A method of forming a photovoltaic devicecomprising: providing an absorption layer comprised of a firstcrystalline semiconductor material having a first conductivity type;epitaxially growing a second crystalline semiconductor layer having asecond conductivity type that is opposite the first conductivity type,wherein temperature of the epitaxially growing the second crystallinesemiconductor layer is 500° C. or less; and forming contacts inelectrical communication with the absorption layer and the secondcrystalline semiconductor layer.
 17. A method of claim 16, wherein thetemperature of the epitaxially growing the second crystallinesemiconductor layer is 200° C. or less.
 18. The method of claim 16,wherein the first conductivity type is p-type and the secondconductivity type is n-type, or the first conductivity type is n-typeand the second conductivity type is p-type.
 19. The method of claim 16,wherein the absorption layer is provided by a semiconductor substratehaving a single crystal crystalline structure, the absorption layerhaving a thickness ranging from 50 nm to 1 mm, and having a dopantconcentration that provides the first conductivity type of theabsorption layer that ranges from 10⁹ atoms/cm³ to 10²⁰ atoms/cm³. 20.The method of claim 16, wherein the absorption layer has a singlecrystal crystalline structure, and the epitaxially growing of the secondcrystalline semiconductor layer produces a single crystal crystallinestructure for the second crystalline semiconductor layer, wherein thesecond crystalline semiconductor layer has a thickness ranging from 2 nmto 2 um, and the second crystalline semiconductor layer has a dopantconcentration that ranges from 10¹⁶ atoms/cm³ to 5×10²⁰ atoms/cm³. 21.The method of claim 16, wherein the epitaxially growing of the secondcrystalline semiconductor layer comprises plasma enhanced chemical vapordeposition (PECVD) from a mixture of silane (SiH₄), hydrogen (H₂) anddopant gasses.
 22. The method of claim 21, wherein the ratio of silane(SiH₄) to hydrogen (H₂) is greater than 5:1.
 23. The method of claim 22,wherein the ration of silane (SiH₄) to hydrogen (H₂) ranges from 5:1 to1000:1.
 24. The method of claim 22, wherein the second conductivity typedopant is n-type, the dopant gasses comprise phosphine gas (PH₃) presentin a ratio to silane (SiH₄) ranging from 0.01% to 10%, or the dopantgasses comprise arsenic gas (AsH₃) present in a ratio to silane (SiH₄)ranging from 0.01% to 10%.
 25. The method of claim 22, wherein thesecond conductivity type dopant is p-type, the dopant gasses comprisediborane gas (B₂H₆) present in a ratio to silane (SiH₄) ranging from0.01% to 10%, or the dopant gasses comprise trimethylboron gas (TMB)present in a ratio to silane (SiH₄) ranging from 0.01% to 10%.